Iran faces off with Western-allied Saudi Arabia for influence in the Middle East. The two regional powers back opposing sides in the conflicts in Yemen and Syria as well as rival political groups in Iraq and Lebanon.
Iran turns to diplomacy amid high regional tensions
"Today, every movement that leads to mutual understanding of Islamic groups and sects is a good thing, and this gathering is an example of the movement for the unification of the Islamic nation," said Khamenei in remarks carried by his official website.
"The scientific movement must accelerate and the Islamic nation must once again reach heights of science and civilisation so that the enemies of Islam and the Americans cannot order around the heads of Islamic countries," Khamenei told a meeting about contributions to sciences.
Tearing up Iran deal: Pakistan counts the cost of Trump move
Iran frequently calls for Islamic unity to help Tehran in its confrontation with its arch-foe, the United States.
Washington on Tuesday pulled out of a nuclear deal with Iran, a move accompanied by the threat of penalties against any foreign firms doing business in Iran.
Tehran denies seeking atomic weapons and says its nuclear programme is for peaceful uses in areas including electricity, agriculture and medicine.
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